1. Field of the Invention
This invention relates to light signal processing systems, and more particularly to arrangements for converting one polarization of light into another.
2. Description of the Related Art
Light sources tend to emit randomly oriented, unpolarized light. Many types of light processing devices are polarization-sensitive, requiring placement of a polarization filter between the light source and the device for operation in the intended mode. This inevitably results in loss of half of the power from the light source, namely that portion of the light power that is in the non-preferred polarization. The lost light power goes into heating losses and often reduces system signal-to-noise and/or efficiency.
One way to recover the energy in the non-preferred polarization is to pass the preferred polarization through a polarizing filter and reflect the non-preferred polarization. The reflected polarization is then rotated to the desired polarization using any type of rotator, which may be a crystal rotator, and summed with the preferred polarization component which passed through the polarizing filter. This may be difficult to achieve, since any phase shift will tend to result in cancellation of the signals being combined.
Thus, what is needed is an improved light polarization transformer.
According to the invention, the randomly phased light source is applied to one end of a polarization-converting optical fiber, and the preferred polarization is extracted from the other end of the fiber. The signal power at the output end of the polarization converter is ideally exactly equal to that at the input end, but of course the actual power will depend on the through loss of the device. In principle, the signal power of the non-preferred polarization is transformed into signal power of the preferred polarization, thereby actually transforming the polarization.
In a preferred embodiment, the fiber of the present invention has a circular end, for input of light having randomly phased polarization, and an elongated end, for output of light having a single principal orientation component. A generally smooth transition from the circular end to the elongated end causes substantially all the light entering the fiber to exit from the fiber without losing light due to reflection or radiation. Thus, light propagating through the fiber is transformed from randomly oriented, unpolarized light into light having a single principal polarization component.
Alternatively, the fiber can have a lobed end, for input of light having first and second mutually orthogonal polarization components, and an elongated end, for output of light having a single principal orientation component. As light propagates through the fiber, the light is transformed from light having mutually orthogonal components into light having a single principal polarization component.
Alternatively, the fiber can be a combination of the two forms described above. That is, the fiber can have a circular section that transforms first into a lobed section with increasing distance from the input port, with the lobed section then transforming into an elongated (e.g., elliptical or ovoid) section with increasing distance from the input port.
Such fibers provide a low-cost and effective device for transforming undesired light polarization into one or more desired polarizations.